The invention relates to a method and apparatus for open-die forging of workpieces in forging presses at high forging temperatures, more particularly steel billets or ferrous alloy billets at forging temperatures between about 800 and 1250 degrees Celsius.
When workpieces, that is, metal billets heated to forging temperature, undergo open-die forging in a forging press, considerable bulging may occur during hammer- or pressure-forging, and cracks may form in the exposed sides of the workpiece. To straighten these undesirable bulges, the workpiece must be turned and the bulges pressed down. These measures may worsen surface quality, and above all they take time. The loss of time often also causes the forging temperature to drop below the acceptable minimum, so that the workpiece must be reheated, so increasing both energy consumption and cost. Lubrication for open-die forging is not known in the prior art.
In extrusion and closed die-forging, on the other hand, it is known for glass to be used as a lubricant. This applies both for high deformation temperatures of about 1100 to 1250 degrees Celsius and for the hot forming range. For example, the periodical "Blech" 11 (1964), volume 9, describes glass or glass-like materials as protection and lubrication during steel forming in pipe and other extrusion presses. The protective and lubricant glass between the hot, plastically deformable metal and the cooler steel of the tools forms a more or less thin film, and slides past the forming tools with the plastically deformed steel. The use of lubricants reduces friction and energy consumption.
The viscosity or plasticity of the glass film reduces the friction coefficient to a value of 0.05, whereas the value would be about 0.12 if the plastic steel were forged without glass lubrication. The heat losses are reduced, and at the same time the tools are protected because the quantity of heat transmitted to the press tools is substantially reduced with glass lubrication, because the thermal conductivity of a viscous or plastic glass film is some 10 to 60 times less than that of an equally thick layer of plastic steel.
German Offenlegungsschrift No. 25 15 222, moreover, describes a method of high-temperature die-forging in which a glass lubricant not more than 25um thick is applied to the surface of the blank. This glass film known in the extrusion art is very thin and therefore tears easily, in which case lubrication is no longer adequate.
An object of the invention is to affect the material flow in the workpiece in open-die forging in a reproducible and operationally reliable manner, and to reduce the total energy consumption for the deformation and heating or re-heating of the workpieces, while at the same time improving the product quality, eliminating in particular bulging and crack formation. A partial object of the invention is also to demonstrate the essential parameters for optimum material flow conditions under operation conditions.